1. Field of the Invention
The present invention relates to a semiconductor memory device and a stored data readout method. In particular, the present invention relates to a semiconductor memory device in which a reference memory cell for generating a reference electric potential or a reference electric current for use in reading out stored data is installed, and a readout method for the stored data.
2. Description of Related Art
As a variable resistance-type nonvolatile memory cell, a STT-RAM (Spin Transfer Torque-Random Access Memory), a PCRAM (Phase Change Random Access Memory), a ReRAM (Resistance Random Access Memory) or the like has been known. In the variable resistance-type nonvolatile memory cell, upon reading out data to be stored in the memory cell, a reference signal (such as reference voltage, or reference current) is used.
As one example of the means for generating this reference signal, a memory cell for use as the reference (hereinafter, referred to as a reference memory cell) has been used. In general, the reference memory cell has the same configuration as that of a normal memory cell, and is disposed in a memory cell array in many cases.
It has been known that the reference memory cell is configured by two memory cells so as to use an intermediate value consequently generated by the cells (intermediate voltage or intermediate current) as the reference signal. In this example, each of reference memory cells for storing data “0” and each of reference memory cells for storing data “1” are used one by one.
A sense amplifier compares a readout signal from the memory cell with the reference signal and discriminates data based upon the sizes thereof.